The present invention relates generally to fiber optic networks, and in particular, to switches for directing optical signals along selected fibers of an optical network.
In fiber optic networks, light signals are transmitted along optical fibers to transfer information from one location to another. A light signal must be accurately entered into an optical fiber, or much of the signal strength will be lost. Modern optical fibers are very small in cross-section, and typically have a fairly narrow acceptance angle within which light entering the fiber should fall to promote efficient propagation of the light signal along the fiber. Therefore, optical switches generally rely on precise and selectable alignment between one or more input optical fibers and one or more output optical fibers. The alignment requirements of modern single mode optic fibers are particularly stringent, as their core diameters are typically as small as 2.0 to 10.0 mxcexc.
In known electromechanical optical switches, the switching operation is often effected by precise movement of the ends of the input fibers relative to the ends of the output fibers, or by accurately moving a mirror to redirect the optical signals to a selected output fiber without moving the optical fibers themselves. Unfortunately, these accuracy and precision requirements substantially increase the cost and decrease the reliability of known optical switches.
Alternative known optical switch structures split the signal and selectively block the undesired optical pathways. Such switches are highly inefficient, requiring repeated signal amplification. Repeated amplification is costly, and also increases the potential for noise and distortion of the original optical signal. These disadvantages are compounded in complex optical switches which provide multiple alternative pathways with simultaneous switching, such as in 2.times.2 switches, N.times.N switches, N.times.M switches, and the like.
A particular challenge with electromechanical fiber optic switches is that they operate as an interface between two data transmission mediums. While the goal of these structures is to provide switching between optical fibers, they will often be actuated by electro-servos. Hence, when switching failures occur, it may be difficult to determine whether the failure lies in an optical component of the network, an optical component of the switch, an electrical component of the switching control circuitry, or an electromechanical component of the switch itself.
U.S. Pat. Nos. 5,867,617 and 5,999,669 to Pan et al. address some of the above problems by proposing Mxc3x97N fiber optic switches utilizing a number of parallelogram-shaped prisms between input and output optical fibers for redirecting optical signals to alternative outputs. It is noted that, while the Pan proposal is useful and operable in a variety of configurations, including 1.times.2, 2.times.2, N.times.N, and M.times.N, with good switching performance, and at an affordable cost, it may suffer from a disadvantage caused by temperature instability.
It is desirable to provide temperature stability for optical switches e.g. of the type as disclosed in the Pan patents, supra. The extremely close tolerances that exist in modern optical switches can be affected in such a way as to cause a reduction in the amount of light transmitted through the switch when the switch is subjected to high ambient temperatures. Such temperatures can exist inside telecommunication equipment cabinets. These temperatures cause the elements within the switch, e.g. the cantilever-type actuators of Pan et al ""699 patent to expand and alter the placement of the components of the switch. While the thermal expansion cannot be eliminated, it would be desirable to minimize its effect.
The invention partly modifies the concept of Pan et al, U.S. Pat. No. 5,867,617, by providing for a linear displacement of the prisms relative to respective optical paths, preferably by linear displacement of prism-supporting means. The provision of the linear displacement alleviates or reduces the effects of thermal expansion on the prism positioning in the respective optical path.
The thermal stability does not result only from the fact that the motion is linear. It results also from the fact that the actuators can be more robust and are not limited to to having to be of a relatively low mass. In the case of a linear actuator as in FIG. 1, the actuator actually slides on the base of the switch. In USP ""617, supra, actuators must lift the entire mass of the cantilever arm and the prism. The present concept can be applied to any linear motion perpendicular or essentially perpendicular to the optical path.
Thus, in accordance with the invention, there is provided an optical switch comprising at least one optical input assembly and at least one output assembly, at least one prism displacement means supporting at least one prism thereon and operable to be moved between a first position and a second position, the first position corresponding to the at least one prism being disposed in an optical path between said at least one input assembly and the at least one output assembly, and the second position corresponding to the at least one prism being disposed out of the optical path, wherein the prism displacement means are linearly movable means for movement between the first and the second position.
It is preferable but not mandatory that the displacement means are mounted for movement in a direction generally perpendicular to the respective optical paths.